1. Field of the Invention
The present invention relates to a voice transducer used for telecommunication by a telephone installed in a motor vehicle or the like.
2. Description of the Prior Art
There are various kinds of voice transducers, in one kind of which, sound waves of a natural voice transmitted through the air are detected by a sensor such as a microphone so as to convert them into electric signals and process them as voice signals and in another of which, vibrations of a natural voice transmitted through the skull of an operator's head are detected by a sensor such as a piezoelectric element so as to process them as electric signals. The former is suitable when the ambient noise is small, and the latter is suitable when the ambient noise is so large that it is difficult to receive sound waves progressing in the air.
FIG. 1 illustrates the structure of a conventional voice transducer of the latter kind. Reference numeral 1 denotes an operator's head. The voice transducer is designated by reference numeral 102. Reference numerals 103 and 104 respectively designate a casing and a damper material. 105 designates a piezoelectric transducer, i.e., a bimorph cell. When the operator utters voice sounds with the head 1 resting on the voice transducer 102, its voice signal S1 is transmitted through the head 1 in the form of mechanical vibration, and it vibrates the voice transducer 102 in contact with the head 1. The vibration is received by the casing 103 of the voice transducer 102, and the received vibration is delivered through the damper material 104 of liquid state to the bimorph cell 105. The bimorph cell 105 oscillates laterally, as indicated by an arrow A, in response to the delivered vibration, to thereby convert the voice signal in the form of mechanical vibration into an electric signal S2.
FIGS. 3A and 3B illustrate states of application of the voice transducer shown in FIG. 1 when it is used as a telephone for a motor vehicle. The voice transducer 102 is installed in a headrest 106 of a seat in the vehicle, with its surface to be contacted with the head 1 being fixed.
In the case of the conventional voice transducer described above, the portion to be contacted with the head is fixed. Consequently, when the operator's head is brought into contact with the voice transducer, the head may not be sufficiently closely fitted to or contacted with the contact portion of the voice transducer, or even if the head is closely fitted to or contacted with the contact portion of the voice transducer, a contact section of the head may not be a place close to the skull but the neck from which the skull is located far away. There will be therefore a decrease in the magnitude of the vibration of the voice transmitted through the skull of the head so as to be input to the voice transducer. In other words, when a short-bodied operator is seated, as shown in FIG. 3A, the voice transducer 102 is located opposite to an upper portion of the head 1 so that the head 1 and the contact portion of the voice transducer 102 can not be sufficiently closely fitted to each other. On the other hand, when a long-bodied operator is seated, as shown in FIG. 3B, the voice transducer 102 is contacted with the operator's neck so that the vibration of the voice transmitted through the skull of the head 1 can not be detected by the voice transducer 102 without difficulty. As a result, there are encountered problem in that the voltage of the voice signal output from the voice transducer 102 is decreased, and the S/N (sound to noise) ratio is deteriorated.
Moreover, in the case of the voice transducer 102 shown in FIG. 1, when the operator's head 1 is brought into contact with the voice transducer 102, the operator's hair 11 (see FIG. 2) is held between the head 1 and the voice transducer 102, so that unfavorable vibration is inevitably generated by friction between the hair 11 and the surface of the casing 103 when the head 1 is moved. The unnecessary vibration is detected by the voice transducer 102 along with the vibration of the voice transmitted from the head 1, resulting in a problem that the S/N ratio of the electric signal output from the voice transducer 102 is deteriorated drastically.